Danios are small schooling fish. There are many different breeds, with different colours and patterns, and many of these breeds are very popular with aquarium hobbyists. Although there are some types of fish that are hard to breed in an aquarium setting, danios breed well on their own. If you suspect your female danio may be pregnant, it's important to find out for sure, so that you can prepare the tank for the baby fish, called fry.
Note the temperature of water in the aquarium. Danio fish usually spawn at temperatures in the low to mid 70s Fahrenheit. If it is much cooler or warmer than this it is unlikely that your danio is carrying roe.
Make sure the fish you are looking at is actually a female. Male and female danios look very much the same, but if the fish in question turns out to be male he could actually be having some serious health problems. Female danios are generally fatter than male danios, whether they are carrying roe or not. In addition, in some species of danios, the females are slightly less colourful and may have fewer stripes, especially on the fins.
Look at your female danio. If she looks wider than normal, she is probably carrying roe. Since danios are not livebearers and female danios never carry fertilised eggs, this is as close as danios get to being pregnant. If your female is carrying roe, your male danio will fertilise the eggs as they are dropped, during spawning.
Watch your fishes' behaviour. Danios like to spawn in the mornings. If your fish chase each other quite a bit at this time, you female may be carrying roe, and the fish may be spawning or near to spawning. In fact, if you see this type of near-violent behaviour between male and female fish, your danios may have already spawned and produced eggs.
Danios will eat their own eggs not long after they have laid them. If you want the danio eggs to hatch, you will have to keep the parents from the eggs. You can do this by laying marbles or a loose-weave net over the bottom of the tank. The eggs will fall through the gaps in these materials and the parents won't be able to reach them. You can then remove the parents from the spawning tank into a different tank. You can also remove the eggs from the tank with the parents in it, but this may be slightly trickier.